The proper care of contact lenses can be viewed as requiring three necessary steps. First, after removal from the eye the lenses must be cleaned to physically remove foreign matter from their surfaces. Second, the lenses must be disinfected. Finally, the lenses must be prepared for insertion into the eye.
In the past, contact lenses have been made of hard polymethacrylates. Proper care of these lenses has required that they be stored in specially developed cleaning and storage solutions to maintain them in good order when not in use. The storage solutions are formulated to disinfect the lenses during the lenses' storage. Many of these cleaning and storage solutions contain chlorobutanol as a preservative which acts to preserve the sterility of the solution.
Recently, a new type of contact lens known as a soft lens has been developed. Soft lenses can be divided into two broad categories, namely hydrophilic lenses and hydrophobic lenses. The care of each of these lenses presents special and different problems.
Hydrophobic contact lenses are usually based on elastic and flexible silicone rubber (polysiloxane), and are generally made from cross-linked dimethyl polysiloxane which is commonly known as Antifoam A. A typical preparation of a hydrophobic silicone contact lens is disclosed in U.S. Pat. No. 3,228,741. Clinical testing of flexible silicone rubber lenses has created a need for cleaning compositions that can be effectively used with these lenses.
In testing the commercially available compositions designed for conventional hard polymethylmethacrylate contact lenses, it has been found that they are not adequate and in some instances detrimental to the successful use of flexible silicone contact lenses. For example, it has been found that the preservative chlorobutanol, present in many commercially available solutions designed for hard polymethylmethacrylate lenses, is adsorbed and concentrated by silicone lenses. This ability of the silicone elastomer to concentrate chlorobutanol could ultimately change the physical and chemical properties of the lenses to make them ineffective in providing visual correction.
Moreover, patients experimentally wearing flexible silicone rubber lenses stored in a chlorobutanol containing storage solution have complained of discomfort. This was found to be directly associated with the high concentrations of chlorobutanol in the silicone lenses thus treated.
The highly hydrophobic nature of the silicone elastomer has prevented their uniform and effective cleaning and wetting by all available conventional cleaners and wetting agents. Thus, various generic classes of organic compounds have been screened including alcohol with varying degrees of acetylation, polysaccharides, lanolin derived nonionic surfactants, ethoxylated sorbitol anhydrides, and vaious cationic, anionic and nonionic detergents, but to date none have been found acceptable.
Hydrophilic soft contact lenses are hydrated gel lenses which can be prepared by copolymerizing hydrophilic organic monomers having an olefinic double bond with a small amount of a cross-linking agent usually having two polymerizable, olefinic double bonds. These lenses are usually based on polyhydroxylated alkyl methacrylates, such as polyhydroxyethyl methacrylate, cross-linked with, for example, a hydroxyethyl dimethacrylate. Usually, there is about one (1) cross-linking molecule for every 200 monomer units. By comparison, the conventional hard contact lens consists of polymethylmethacrylate cross-linked with hydroxyethyl dimethacrylate. The absence of a hydrophilic OH group in conventional hard lenses accounts for the tremendous difference in behavior of the two materials.
Hydrated gel lenses can contain the following materials: (1) hydroxyethylmethacrylate (HEMA) or its analogues, (2) ethylene-glycol dimethacrylate (EGMA) or its analogues, (3) polymethylmethacrylate (PMMA) or its analogues, (4) polyvinylpyrrolidone (PVP), (5) traces of the respective monomers, (6) traces of inhibitors such as hydroquinine, (7) traces of catalysts such as benzoyl peroxide, and (8) water. A more detailed description of hydrated gel lenses is found in U.S. Pat. Nos. 2,976,576; 3,220,960; 3,361,858; 3,408,429; 3,496,254; and 3,499,862.
Soft contact lenses of the hydrated gel type have a number of properties which complicate their effective care. For example, the hydrophilic OH groups of the lenses attract and hold large amounts of water in the plastic, and this leads to difficulties in cleaning and sterilizing the lenses. Further difficulties in caring for hydrated gel lenses occur because these lenses complex and concentrate chlorobutanol, benzalkonium chloride, thimerosal, phenylmercuric nitrate and other preservatives found in solutions for conventional lenses. Generally, these preservatives are inactivated in the complexed state. Also, if concentrated preservatives are released too rapidly at the cornea, they may cause chemical burns. Thus, solutions and cleaners now available for conventional hard lenses cannot be used with gel lenses.
Another type of contact lens which has recently been developed is the semi-rigid, gas permeable, cellulose acetate butyrate lens. These lenses are somewhat more flexible than conventional hard polymethacrylate lenses, but less flexible than the common soft lenses.
Cellulose acetate butyrate lenses are based on organic cellulose esters produced by the reaction between chemical cellulose and the appropriate acid and anhydrides in the presence of a suitable chemical catalyst. For example, cellulose acetate, prepared by treating cotton linters with sulfuric acid and acetic acid, may be esterified with a mixture of butyric acid, acetic anhydride, and a small amount of a concentrated sulfuric acid catalyst. The reaction is allowed to proceed virtually to completion, so that the cellulose fiber structure completely disappears to produce a uniform, homogeneous product. A typical preparation of a cellulose acetate butyrate contact lens is disclosed in U.S. Pat. No. 3,900,250.
Cellulose acetate butyrate lenses have a number of advantages. Because of their flexibility, they are less irritating upon contact with the eye during the initial stages of adaptation by the wearer and during permanent wearing. Furthermore, while the lens material is flexible to the extent it is practically unbreakable in normal use, it has sufficient rigidity to be easily machined. However, care of these lenses is complicated by the fact that, like hydrated gel type lenses, they tend to complex and concentrate certain preservatives, such as benzalkonium chloride, commonly used in solutions for conventional hard lenses.
The methods currently used in caring for hydrophilic gel lenses generally include the following: (1) boiling in saline; (2) treating with 3% hydrogen peroxide; (3) rinsing with "sterile" saline; and (4) storing in disinfecting solutions. Each of these methods, however, have numerous disadvantages.
For example, boiling in saline kills pathogens but does not kill spores. Another disadvantage of boiling is that it is not convenient for patients to carry the boiling devices with them wherever they go. Further, proteins and other materials may be denatured and deposited on or in the lens matrix if the lenses are not adequately cleaned prior to boiling. The effects of boiling on soluble or water dispersible proteins are similar to the coagulating and insolubilizing effects of heat on egg whites. Thus, once these deposits are allowed to accumulate on the lenses, substantially more effort is required to clean them.
Commercial hydrogen peroxide has satisfactory germicidal activity, but its use also has a number of disadvantages. Commercial hydrogen peroxide has a pH of about 3 and it is therefore necessary to treat the lenses with sodium bicarbonate solution to neutralize the high acidity before the lenses can be worn safely. A major concern, however, is the ever present possibility that the patient will forget to neutralize and dilute the hydrogen peroxide with sodium bicarbonate solution prior to inserting the lens. further, the cleaning action of hydrogen peroxide is no better than that achieved with water or isotonic salt solutions. In fact, hydrogen peroxide, because of its oxidative chemical reactivity, can denature and precipitate proteins.
Rinsing lenses with unpreserved and supposed "sterile" saline solution delivered from a large multiple dose bottle, is far from adequate in sterilizing lenses.
Experimental isotonic disinfecting solutions of two basic types are currently available. One solution contains 0.001% thimerosal, 0.1% disodium ethylenediaminetetraacetate, and 0.005% chlorhexidine gluconate. Another solution has the same composition except that the chlorhexidine gluconate is replaced with a like amount of dodecyl triethanolamine hydrochloride. Both of these solutions have drawbacks.
Chlorhexidine and dodecyl triethanolamine are not only inactivated by many peptides, proteins and fatty substances of natural origin bearing a net negative charge, but cause the formation of insoluble precipitates. To a lesser extent, this same phenomenon may occur when the negatively charged thimerosal ion reacts with proteins bearing a net positive charge. Further, neither solution is ideal when prolonged wearing comfort, complete sterility reliance and lack of allergic response are considered. Although these solutions have been tested for their cleaning efficiency, they fall significantly short of accomplishing this objective when used on a routine basis. This is not surprising since neither of these solutions was specifically formulated for this purpose.
U.S. Pat. No. 3,882,036, issued May 6, 1975 and U.S. Pat. application Ser. No. 377,430, filed July 9, 1973, now U.S. Pat. No. 3,954,644, disclose compositions suitable for cleaning hard and soft contact lenses. The compositions therein disclosed, particularly those containing poly(oxypropylene)-poly(oxyethylene) block copolymers, have proven to be very effective for cleaning flexible silicone lenses.
The disclosed cleaning compositions, however, are not as effective as would be desired with respect to removal from lenses, particularly cellulose acetate butyrate and soft lenses, of cholesterol like materials. If such material is not thoroughly removed, it may build up on or in the lenses, imparing their optical clarity and thus their usefulness. Furthermore, a build up of such materials can affect the wetability of the lenses, thus decreasing wearer comfort.
In addition, cholesterol like material remaining in or on the lenses may eventually inactivate even the best germicidal agents, and serve as a growth medium for a variety of microorganisms. While many germicidal chemicals, in appropriate concentrations, are effective in disinfecting new lenses, the same chemicals do not necessarily disinfect a lens which has been worn repeatedly and improperly cleaned.
The necessity for proper cleaning of all types of contact lenses, particularly soft silicone or hydrophilic gel lenses, is readily apparent. Single compositions suitable for use with any type of contact lenses clearly would be desirable and no compositions heretofor available are totally suitable for routine cleaning of soft lenses.